1. Field of the Invention
The invention relates to a liquid crystal composition and a liquid crystal display device. More specifically, the invention relates to a liquid crystal composition suitable for use in an active matrix (AM) device, and a liquid crystal display device containing the composition.
2. Related Art
A liquid crystal display device (which is a generic term for a liquid crystal display panel and a liquid crystal display module) utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal composition, and as an operating mode of the liquid crystal display device, such various modes have been known as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, and so forth.
In a TN mode, an STN mode and so forth, a composition having a positive dielectric anisotropy is used. In a VA mode, a composition having a negative dielectric anisotropy is used. In an IPS mode, a composition having a positive or negative dielectric anisotropy is used. An ECB mode, an IPS mode, a VA mode and so forth using a composition having a negative dielectric anisotropy among these operation modes are such an operation mode that utilizes the fact that liquid crystal molecules are oriented perpendicular to the electric field, and in particular an IPS mode and a VA mode are such a mode that utilizes electrically controlled birefringence (ECB) and have been known as being capable of improving a narrow viewing angle, which is a defect of the conventional operation mode, such as a TN mode and an STN mode.
Various compositions have been investigated that contain, as a liquid crystal compound having a negative dielectric anisotropy capable of being used in a liquid crystal display device of these operation modes, such a liquid crystal compound that hydrogen on a benzene ring is replaced by fluorine (see, for example, JP H02-503441 A/ 1990).
The liquid crystal compound is a generic term for a compound having a liquid crystal phase such as a nematic phase, a smectic phase and so forth, and also for a compound having no liquid crystal phase but being useful as a component of a composition. The content ratio of the component is calculated based on the total weight of the liquid crystal composition. The liquid crystal compound herein is a compound represented by formula (A). The compound may be an optically active compound:

In formula (A), Ra and Rb are independently are hydrogen, alkyl, alkoxy, alkoxyalkyl, alkoxyalkoxy, acyloxy, acyloxyalkyl, alkoxycarbonyl, alkoxycarbonylalkyl, alkenyl, alkenyloxy, alkenyloxyalkyl, alkoxyalkenyl, alkynyl, alkynyloxy, cyano, —NCS, fluorine or chlorine. These groups have 10 or less carbons. In the group having 1 to 5 carbons, arbitrary hydrogen may be replaced by fluorine or chlorine, and the total number of the replaced fluorine and chlorine is 1 to 11.
In formula (A), ring B is 1,4-cyclohexylene, 1,4-phenylene, pyrane-2,5-diyl, 1,3-dioxane-2,5-diyl, pyridine-2,5-diyl, pyrimidine-2,5-diyl, decahydronaphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl or naphthalene-2,6-diyl. In ring B, arbitrary hydrogen may be replaced by fluorine or chlorine. In ring B, the total number of the replaced fluorine and chlorine is 1 to 4. In the 1,4-phenylene, arbitrary one or two hydrogens may be replaced by cyano, methyl, difluoromethyl or trifluoromethyl.
In formula (A), Y represents a single bond, —(CH2)2—, —COO—, —OCO—, —CH2O—, —OCH2—, —CF2O—, —OCF2—, —CH═CH—, —CF═CF—, —(CH2)4—, —(CH2)3—O—, —O—(CH2)3—, —CH═CH—(CH2)2—, —(CH2)2—CH═CH—, —(CH2)2CF2O—, —OCF2(CH2)2—, —(CH2)2COO—, —(CH2)2OCO—, —COO(CH2)2—, —OCO(CH2)2—, —CH═CH—COO—, —CH═CH—OCO—, —COO—CH═CH— or —OCO—CH═CH—.
In formula (A), n represents 1, 2, 3 or 4.
However, the compound having a hydrogen on a benzene ring replaced by fluorine exemplified in JP H02-503441 A/1990 is poor in compatibility with other liquid crystal compounds in a low temperature range, and in the case where a liquid crystal composition is formed with the compound, the composition cannot be used in a low temperature range in some cases.
A liquid crystal composition containing a compound having 2,3-difluorophenylene has been disclosed (see, for example, JP H10-251644 A/1998). However, a liquid crystal composition containing a compound having 2,3-difluorophenylene cannot have a small optical anisotropy (Δn) (for example, 0.07 or less) in some cases. Furthermore, JP H10-251644 A/1998 contains no disclosure relating to a minimum temperature of a nematic phase of the liquid crystal composition.
EP 0 474 062 A2 (U.S. Pat. No. 5,384,065) discloses a compound having 2,3-difluorophenylene in the structure thereof as a compound suitable for a liquid crystal display device, but contains no disclosure of the liquid crystal compound having the specific structure as a component of the invention, in which hydrogen on a benzene ring is replaced by chlorine and fluorine.
A composition containing a liquid crystal compound having a specific structure, in which hydrogen on a benzene ring is replaced by chlorine and fluorine, has been disclosed (see, for example, International Publication 98/23561/1998 (U.S. Pat. No. 6,329,027). However, a composition containing the compound does not have a high clear point and has a large viscosity.
A liquid crystal display device having such an operation mode as an IPS mode and a VA mode still has a problem as a display device in comparison to a CRT, and the characteristics thereof are demanded for improvement.
The liquid crystal display device driven in an IPS mode or a VA mode is constituted mainly by a liquid crystal composition having a negative dielectric anisotropy, and in order to improve further the characteristics, the liquid crystal composition is demanded to have the following characteristics (1) to (5), i.e., (1) a wide temperature range of a nematic phase, (2) a low viscosity, (3) a suitable optical anisotropy, (4) a large absolute value of dielectric anisotropy, and (5) a large specific resistance.
The temperature range of a nematic phase relates to a temperature range where the liquid crystal display device is used, and a liquid crystal display device containing a liquid crystal composition having a wide temperature range of a nematic phase as in the item (1) has a wide temperature range where the liquid crystal display device can be used.
A liquid crystal display device containing a liquid crystal composition having a small viscosity as in the item (2) has a short response time. A liquid crystal display device having a short response time can be favorably used for displaying a moving image. Furthermore, upon injecting the liquid crystal composition into a liquid crystal cell of the liquid crystal display device, the injection time can be reduced to improve the workability.
A liquid crystal display device containing a liquid crystal composition having a suitable optical anisotropy as in the item (3) has a large contrast.
A liquid crystal display device containing a liquid crystal composition having a large absolute value of dielectric anisotropy as in the item (4) reduces a threshold voltage, decreases a driving voltage, and reduces an electric power consumption.
A liquid crystal display device containing a liquid crystal composition having a large specific resistance as in the item (5) increases a voltage holding ratio and increases a contrast ratio. Therefore, such a liquid crystal composition is preferred that has a large specific resistance in the initial stage and has a large specific resistance even after using for a long period of time.